There are known a coaxial line and a coplanar line as high-frequency transmission lines for transmission of a high-frequency signal such as a microwave or millimeter wave signal.
In a high-frequency transmission line, a single transmission line may be achieved by connecting different types of transmission lines. In such a case, it is necessary to connect the different types of transmission lines yet still suppressing deterioration in transmission characteristics. For example, non-patent literature 1 discloses a connection structure of an arrangement in which the axis of a signal line between a coaxial line and a coplanar line is continuously shifted toward the coplanar line from the center of the coaxial line.